NiCrAlY and NiCoCrAlY type coatings derive their protective capability from their ability to form (and repeatedly reform) a thin layer of alumina on their outer surface. It is this alumina layer which resists oxidation and hot corrosion attack. Oxygen active elements such as yttrium are added to the coatings to promote adherence of the alumina and to retard its tendency to spall. Hafnium can also be added to improve alumina adherence.
The prior art did not anticipate this invention, apparently because of the sequence of developments in the superalloy and coatings field. In the beginning, superalloys were used in the so called equiaxed form and were used either uncoated or coated with the simple coatings such as aluminide coatings. As performance requirements increased, there were parallel improvements in the superalloy and coating areas; in the superalloy area substantial improvements were obtained through the development of directional solidification in which the superalloy is caused to solidify with elongated grains which are crystallographically oriented. This is described in U.S. Pat. No. 3,260,505 which is assigned to the assignee of the present application. In the coating area at about the same time the MCrAlY type overlay coatings were developed. Such coatings are substantially more protective than the previously used aluminide coatings. U.S. Pat. Nos. 3,542,530, 3,676,085, 3,754,903 and 3,928,026 relate to MCrAlY overlay coatings.
Despite the promise of directional solidification, significant problems were encountered in the area of transverse ductility. It was found that the grain boundaries separating the orientated elongated grains were notably weak in the direction transverse to the axis of elongation. This problem was overcome through the addition of small amounts of hafnium to the alloy (0.5-2% Hf) as described in U.S. Pat. No. 3,711,337 which is also assigned to the present assignee.
Because of the development of hafnium modified directional solidified articles and MCrAlY overlay coatings in the same laboratory at about the same time, it was natural that they would be used in combination with each other. It is also natural that highly demanding applications, specifically first stage turbine blades, would involve the use of both hafnium modified directional solidified substrates and MCrAlY overlay coatings.
The MCrAlY coatings were optimized, as to composition, on directionally solidified hafnium containing columnar grain substrates. It was found that yttrium in levels greater than about 0.5 or 0.7 percent by weight produced undesirable low melting point phases at the coating substrate interface, apparently phases based on yttrium and hafnium.
An advancement beyond columnar grain articles is the use of similar but more advanced solidification techniques to produce single crystal articles as described in U.S. Pat. No. 3,494,709. Such single crystal articles are free from internal grain boundaries and hence do not suffer from the transverse ductility problem encountered in the case of columnar grain articles. Consequently, there is no need to add hafnium to tne alloys for ductility improvements and in fact it has been found desirable to eliminate hafnium so as to improve the heat treatability of single crystal superalloy articles. This subject is discussed in U.S. Pat. No. 4,116,723 which is assigned to the present assignee.
The present invention arises from the belated appreciation that the hafnium in the substrate material played a significant role in enhancing coating performance as well as in improving transverse ductility and that the previously developed NiCrAlY and NiCoCrAlY coatings can be significantly improved.